1. Field of the Invention
The invention relates to a lighting device for stimulating plant growth, comprising at least one solid state light source, suitable for emitting light of at least one wavelength within a predetermined wavelength range, wherein the at least one solid state light source is adapted for connection to an electrical network.
2. Description of the Related Art
The photosynthesis process for plant growth at deep-red light is based on the absorption of light with a peak wavelength of 662 nm by chlorophyll A and light with a peak wavelength of 642 nm by chlorophyll B. In general, red Light Emitting Diodes (LEDs) have the highest efficiency among the LEDs, i.e. a relatively high emitted power per Watt, wherein the non-emitted power is converted into heat.
However, the light yield of red LEDs drastically decreases with an increase in junction temperature, i.e. the temperature at the junction of P-N semiconductor materials of the LED. For this reason, LEDs are mostly mounted on a cooling element.
It has been shown that with a temperature increase of the LED semiconductor junction from 20° C. to 75° C., the light yield may decrease with at least a factor of two.
Red LEDs of a.o. the AlGaInP (Aluminum Gallium Indium Phosphide) type are considerably more sensitive to temperature changes than for example green and blue LEDs, for example of the InGaN (Indium Gallium Nitride) type. An advantage of AlGaInP LED material is that the life span is extremely long, i.e. under normal conditions such material has a life span of 100,000 hours.
The Japanese patent applications IP 2003009662 and JP 2004113160 and the U.S. Pat. No. 6,921,182 describe the application of LEDs for plant growth. Herein, among other, use is made of red LEDs. In all of these patent publications low-power LEDs are used, in this case with a power of approximately 60 mW. The temperature of the carrier of these LEDs may rise rapidly, as a result of which the efficiency and the scope of application remains largely below the current assimilation lighting with gas discharge lamps adapted for plant growth. For example, for taking over the task of a standard sodium lamp of 600 W 10,000 of such LEDs would have to be used.
Hence, large-scale application of LEDs in greenhouses demands large numbers of power LEDs of 0.5 W or more. Such LEDs produce a lot of heat. Therefore, the carriers for such LEDs are provided with a part which is adapted for emitting the heat generated by the power LEDs. A well-known technique for semiconductor cooling is mounting the LEDs on a plate such that they make a good thermal connection with the plate. On its turn, the plate is connected to a device wherein the absorbed heat can be emitted. In such a heat release system the heat generated is conducted by the plate using heat conduction to the location for heat emission and is subsequently dissipated into the surroundings using radiation. Although such a heat emission system reduces the temperature of a carrier, this is generally still insufficient.